1. Field of the Invention
This invention relates to a variable inductor, and more particularly to a variable inductor which uses a circuit constituted of an active element and a plurality of interconnected inductors to change inductance.
Additionally, this invention relates to an oscillator having an inductor, and a radio terminal having an inductor, and also relates to improvements in a circuit designing technology of an amplifier and a radio terminal and a gain varying method of the amplifier.
2. Description of the Related Art
Generally, in order to vary characteristics of an electronic circuit, characteristics of an active element or a value of a passive element included in the circuit is changed. For the active element, the characteristics of the active element can be changed by changing a bias voltage applied to the active element. For the passive element, a passive element, for example, a variable resistive element can be easily realized by using ON resistance of a MOSFET, and a variable capacitive element can be easily realized by using a pn junction.
For an inductor as a passive element, it is generally considered difficult to vary inductance while maintaining good characteristics. A method which uses an active element to constitute an inductor and varies inductance is disclosed in ELECTRONICS LETTERS 2nd January, Vol. 28, No. 21, pp. 78 to 80, 1992. However, there is a problem of bad characteristics of noise or distortion because of use of the active element for the inductor.
Thus, various technologies have been presented, which vary inductance without constituting an inductor of an active element. However, all of these technologies have problems in practical application. For example, Jpn. Pat. Appln. KOKAI Publication No. 8-162331 discloses a method which inserts a switch into the middle of an inductor, and turns the switch ON/OFF to change inductance. This method has a problem that ON resistance of the switch deteriorates performance of the variable inductor.
Jpn. Pat. Appln. KOKAI Publication No. 2000-223317 discloses a method which physically changes a shape of an inductor by a laser beam. This method necessitates a physical adjustment of the inductor after it is manufactured. Thus, there are problems of high manufacturing costs and a difficulty of changing inductance in a situation where a circuit is operated.
Jpn. Pat. Appln. KOKAI Publication No. 7-320942 discloses a technology which constitutes a variable inductor by using interconnection of a plurality of inductors. According to this method, a shape of the inductor is physically changed in order to change a coupling coefficient. Thus, there are problems of miniaturization and low costs of a circuit which constitutes the variable inductor.
Further, D. R. Pehlke et al. have presented, in U.S. Pat. No. 5,994,985, a technology which uses a directional coupler to separate an input signal into two, and controls amplitudes and phases of signals flowing through two interconnected inductors to change inductance. However, since the directional coupler is not generally suited for integration, there is a problem that it is difficult to realize a variable inductor by an integrated circuit.
By realizing the variable inductor, it is also possible to control even a voltage-controlled oscillator (VOC) which includes an LC resonant circuit (inductor: inductance L, capacitor: capacity C). An oscillation frequency (f) of the voltage controlled oscillator which includes this LC resonant circuit is generally represented by f=½[2π(LC)½]. If the inductance l or the capacity C is controlled, the oscillation frequency (f) is controlled. However, in the conventional LC resonant circuit, it is difficult to realize a variable inductor as described above. Generally, therefore, the capacity C is varied, for example, a reverse bias application voltage to a pn junction diode is changed to vary the capacity C and, by changing this capacity, the oscillation frequency is changed.
If such a voltage controlled oscillator is formed as an integrated circuit on a semiconductor substrate, i.e., IC formation, a parasitic capacitance, e.g., a parasitic capacitance of the inductor, a drain parasitic capacitance of a MOS transistor, a gate parasitic capacitance of the MOS transistor or the like, is generated. Generation of such a parasitic capacitance is inevitable, and thus there is a problem that such a parasitic capacitance reduces a fluctuation width in a variable capacity C of the LC resonant circuit. For example, if a fluctuation amount of the capacity C is ΔC, a parasitic capacitance amount is added to a denominator as a non-fluctuation portion while a design fluctuation rate is assumed to be ΔC/C. Thus, in practice, there is a problem that the fluctuation rate becomes small, i.e., ΔC/(C+parasitic capacitance). Though it is dependent on a circuit design or the like, if the capacity C and the parasitic capacitance are about equal, a change rate is reduced to about ½.
Because of the presence of such a parasitic capacitance, a ratio of the capacity value of the LC resonant circuit occupied by the variable capacity C is inevitably reduced. Thus, compared with a change rate of the variable capacity C, a change in the capacity value of the LC resonant circuit is reduced and, consequently, a variable range of an oscillation frequency is narrowed.
Today, however, a frequency band used for a portable telephone, a radio LAN device or the like has been widened, and there is a case where a plurality of frequency bands are dealt with by one device. Thus, there has been an increase in demand for expansion of a change width of the oscillation frequency. From this viewpoint, realization of a variable inductor is desired.
The variable inductor can be applied to an amplifier provided with an inductor. For example, in an amplifier which comprises the inductor for degeneration, if its inductance is reduced, distortion characteristics are deteriorated while a gain and noise characteristics of the amplifier are improved. Conversely, if the inductance is increased, a gain and noise characteristics are deteriorated while distortion characteristics are improved. Because of this trade-off relation, an inductance value is decided to obtain desired characteristics when the amplifier is designed.
Generally, realization of low distortion characteristics while maintaining a high gain and low noise characteristics is dealt with by increasing the amount of a current. In the amplifier used for a receiver of a radio terminal, characteristics necessary for the amplifier are different depending on a sized of a received signal. Generally, since it is considered important to amplify a signal at low noise if the received signal is small, a good gain and good noise characteristics are required of the amplifier. On the other hand, if the received signal is large, good distortion characteristics are required.
In the conventional amplifier which comprises the inductor for degeneration, since inductance is fixed, the amount of a supplied current is controlled in order to change the characteristics of the amplifier. That is, to improve distortion characteristics, a current is controlled so as to increase the amount of a supplied current. However, an increase in the amount of a current made to change the characteristics of the amplifier creates a problem of increased power consumption.
Further, there is disclosed a circuit example of a variable gain amplifier in Dual-Band High-linearity Variable-Gain Low-Noise Amplifier for Wireless Applications K. L. Fong, “Dual-Band High-Linearity Variable-Gain Low-Noise Amplifier for Wireless Applications,” IEEE ISSCC99, pp 224 to 225, 1999. In this variable gain amplifier, a first-stage common emitter circuit constituted of a first transistor Q1 is always operated, and gain switching is realized by switching second to fourth transistors Q2 to Q4 which constitute a first-stage common base circuit. Since the first transistor Q1 is operated, input impedance is not greatly changed even if a gain is switched. However, a fixed current is always consumed, and distortion characteristics are substantially constant.
However, in the conventional circuit disclosed in Dual-Band High-Linearity Variable-Gain Low-Noise Amplifier for Wireless Applications K. L. Fong, “Dual-Band High-Linearity Variable-Gain Low-Noise Amplifier for Wireless Applications,” IEEE ISSCC99, pp 224 to 225, 1999, there is a problem that a large current is consumed even when a gain is low, and distortion characteristics are about similar to those when the gain is high. To realize an amplification stage of a basically high gain and good distortion characteristics, current consumption is necessary to a certain extent. In the case of a gain which is not so high or to attenuate a signal, it is possible to realize an amplification stage of good distortion characteristics without consuming a current so greatly. However, if a plurality of amplification stages are switched, there is a problem of a change in input impedance.
As described above, the variable inductor of the conventional art has a problem in electric characteristics, and there is a problem that it is difficult to realize miniaturization, low costs and an integrated circuit.
In the radio terminal such as a portable telephone, there is a strong demand for making an adaptive characteristic change of amplifier characteristics in accordance with a received signal level, while lower power consumption is similarly demanded strongly. In the amplifier which uses a fixed inductor for degeneration, the only way to improve distortion characteristics is to increase the amount of a current, which brings about an increase in power consumption.